1. Field of the Invention
The present invention relates to a laminated heat exchanger used for the cooling cycle and the like of an air conditioning system for vehicles, constituted by laminating tube elements and fins alternately over a plurality of levels and, in particular, it relates to a laminated heat exchanger that employs the so-called 4-pass system, with each tube element being provided with a pair of tanks formed on one side (i.e. at one longitudinal end) so that heat exchanging medium passes through the tube element on two round-trips as it travels from the intake port to the outlet port.
2. Description of the Related Art
The so-called 4-pass system laminated heat exchanger is constituted, as disclosed in Japanese Unexamined Patent Publication No, S63-3153, for instance, by laminating tube elements and fins alternately over a plurality of levels with each tube element being provided with a pair of tanks on one side. The two tanks in this pair communicate with each other via a U-shaped passage and the tank portions in adjacent tube elements are bonded so as to form two tank groups extending in the direction of the lamination. One of the tank groups is partitioned in the middle to divide the inside into two communicating areas and, as shown in FIG. 7, an intake port 20 is provided in one of the communicating areas 22 and an outlet port 21 is provided in the other communicating area 23. Thus, the heat exchanging medium that flows in through the intake port 20 travels through the first and second passes which are constituted by the tube elements located toward the intake port from the partitioning portion. It then travels through the third and fourth passes which are constituted by the tube elements located toward the outlet port from the partitioning portion to flow out through the outlet pert 21.
However, if the heat exchanging medium used is a coolant, the coolant becomes gradually gassified during the process of heat exchanging and expands. Therefore, in the 4-pass system heat exchangers of the prior art, in order to secure enough cross-sectional area in the passage, fewer tube elements are located toward the intake port from partitioning portion than toward the outlet port. However, from research conducted by this inventor, it has been learned that if the outlet port for heat exchanging medium is provided at one end in the direction of lamination of the tube elements, the temperature of the tube elements in the vicinity of the partitioning portion (the tube elements separated from the outlet port 21 that constitute area B in FIG. 7) among the tube elements constituting the third and fourth passes, increases. As a result, an even temperature distribution over the entire heat exchanger cannot be achieved. This is because when identical tube elements are used for lamination, heat exchanging medium mainly flows through the tube elements nearest the outlet port and it does not easily flow through the tube elements around the partitioning portion.